Custom databases (Salsa support)

[brendanzab]

It would be nice to be able to define custom Files-style databases depending on the needs of a project.

• People might want to use something like Salsa to manage their source files.
• When starting off building a language, a 'SingleFile' database (that only took a single file) might be a better solution than Add emit_single, which emits a Diagnostic given a single FileMap #56.
• Others might want to use a more compact span representation than the one used currently. For example compressing it, as is done in rustc.

The language-reporting fork defines ReportingSpan and ReportingFiles traits which might be useful for inspiration.

[brendanzab]

Inching towards a Files trait here: https://github.com/brendanzab/codespan/compare/traits

Still does not compile though - working through how to pull everything out nicely. The crate structure is a tad awkward though - now codespan is only depended on in order to implement Span and Files.

[brendanzab]

I think it'd probably also be nice to implement the LSP support in terms of these traits too.

[brendanzab]

It would also be interesting if we could have a subtrait that also provided URLs for codespan-lsp, which would mean we wouldn't have to supply the correlate_file_url closure.

[Marwes]

Getting some sort of trait in would be nice before upgrading for me as the lack of being able to take individual Arc<FileMap> out makes it impossible to work with Files concurrently (reading from one file while adding others).

I can take https://github.com/brendanzab/codespan/compare/traits and get it working but I don't really know if we must/should make any additional changes to the traits.